The present invention concerns a laser joining method of joining different workpieces made of plastic or joining plastic to other materials, where the top workpiece facing the laser source is made of a material that is transparent for the laser beam, and the second workpiece is made of a material that absorbs the laser beam, so that the contact surfaces of the two workpieces adjacent to one another are fused and bonded to one another in the subsequent cooling under pressure. This invention also concerns a device for carrying out this method.
This method is essentially known from U.S. Pat. No. 4,636,609, for workpieces made of plastic. Meanwhile, there are numerous other patents describing various applications of this method as well as the equipment for carrying out this method in joining specially designed workpieces. For example, International Patent WO 95/26869 as well as the publications cited in the respective Search Report can be mentioned here.
With these known methods, plastic sheets, films and injection molded parts are welded by sharply bundled laser beams, using a laser beam focused in a spot for welding and fusing the plastic materials. The path of the weld is implemented by a programmable beam guidance of the laser beams or by moving the workpieces. So far, there is no effective method of joining surfaces or finely structured objects, such as those needed as part of micro-mechanics or micro-systems engineering, where only the desired zone is to be bonded. One possible alternative to this technical desire would be such a method where the entire surface is bonded by two-dimensional scanning of a point-source laser beam. One disadvantage of this method is that the process time is greatly increased by fine scanning. Another problem is the quality of the joint seam, which is influenced by the scanning distance as well as the scanning rate of the laser spot and the control performance of the laser power. A high-quality joint requires a dense scanning distance and a low scanning rate because of the dynamic properties of the motion system. At high rates of movement, the joint seam can be very problematical. This often leads to unsatisfactory joining results, in particular at the turning point, where there is a boundary line between the joined area and the unjoined area. Another disadvantage of two-dimensional joining with point-focused laser beam is the distribution of the beam intensity, because the gaussian intensity distribution causes irregular melting.
U.S. Pat. No. 4,069,080 describes a method and a device where a laser line is focused on a plastic by using a CO2 laser and appropriate optical equipment, and thus a bag is produced by welding two film halves.
Therefore, the object of the present invention is to propose a possible method of producing a high-quality two-dimensional joining of objects placed one on top of the other, with at least one body being made of plastic, with a well-defined joining zone.
This object is achieved according to this invention. The present invention is based on the idea that to join the workpieces in a certain joining area, a mask of a material that is opaque to the laser beam is arranged between the laser source and the workpieces to be joined, the structure of said mask being greater than the wavelength of the laser beam used. The term xe2x80x9cjoiningxe2x80x9d as used here is understood below to refer to welding, fusing, gluing, bonding or the like. In addition, the laser source is aimed at the contact surface in such a way as to form a line on it. In addition, the laser beam and the workpieces are moved relative to one another. Essentially, a Nd:YAG laser, a CO2 laser or a semiconductor laser with laser beams having a wavelength of 0.7 to 10.6 xcexcm can be used, but appropriate optical measures must be taken to ensure that a linear laser beam is provided at the contact surface. The workpieces are pressed together by the inherent pressure or by means of a suitable clamping device by a known method. A mask, e.g. in the form of a thin metal plate or a metal layer vaporized onto a carrier, is applied to the workpieces; said mask has recesses through which the laser beam can pass in accordance with the structures to be joined. The structures may be less than a millimeter in size. As long as the dimensions of the fine structure of the mask amount to 5-10 times the wavelength of the beam used, there will not be any motion effect that would have a negative effect on the laser results. The laser beam penetrates essentially at a right angle or at an angle which has only an insignificant effect on the boundary line of the joining area, through the openings in the mask and the top workpiece, penetrating as far as the contact surface between the two workpieces. The beams striking the mask are reflected back. The possible joint seam structures are determined mainly by the mask and the thermal properties of the material. Due to the relative movement between the laser beam and the workpieces, which can be implemented by moving the laser source or the workpieces or by a suitably controlled laser beam guidance device, the laser beam penetrates through the openings in the mask either continuously or completely (in the case of small joint areas with same line width with the joining area), yielding a correspondingly structured joining of the workpieces at the desired joining zones in the contact area. Depending on the arrangement and application, the mask with the workpieces can be moved relative to the laser beam or the workpieces can be moved together with the laser beam and the mask.
This method thus makes it possible to join plastic sheets, injection molded parts or films to one another or to other materials in a short cycle time and with a high quality or to accurately join workpieces having a fine structure with a flat seam without damaging the structuring on the desired zone. Thus, for example, linear contours of any desired shape, which previously had to be worked using a point-source laser beam, can be implemented easily due to the linear design of the laser beam by passing it once over a mask of a suitable design.
The laser beam is preferably guided perpendicular to the joining area of the contact surface, to achieve the most accurate possible imaging of small structures in particular and to prevent unnecessary shadow effects.
The workpieces are advantageously pressed together at a pressure above the inherent pressure so that they are in good thermal contact with one another and are held together with a joining pressure because of the thermal expansion of the molten material during the melting operation due to the laser treatment.
To be able to guide or stop the spread of the liquid melt by additional structuring, a projection may be provided on one workpiece in the direction of the other workpiece adjacent to the joining area according to another advantageous embodiment, so that when the two workpieces are pressed together, a cavity is formed between the projection and the contact surfaces of the workpieces to be joined. As an alternative, such projections may also be provided on the workpieces. As soon as the joining area has been irradiated by the laser beam, the liquid melt is diverted into this cavity, because the pressure is much lower inside this cavity than in the contact area where the two workpieces come in contact under pressure. This makes it possible to prevent an unwanted direction of flow of the melt into areas that are to remain free. This is a great advantage especially with microminiature parts. According to another preferred embodiment of the method according to this invention, the relative velocity between the laser source and the workpieces is chosen to be as high as possible, depending on the maximum laser power needed to achieve the required melting point. This results in an optimization with the goal of imaging the mask structure sharply on the desired joining zone by applying the thermal energy required for the melting process to the melting zone only briefly. Thus, the molten material cools immediately after the laser treatment and solidifies before the adjacent material begins to flow.
The line width of the laser beam is advantageously selected to correspond to the width of the joining area. This can be achieved by one or more laser sources arranged side by side in accordance with the power of the laser source. With an increase in the capacity of the laser source, the size of the joining area which is irradiated with the laser all at once can also be increased. This is especially applicable to small joining areas in the cm2 range. Here again, CO2 lasers or semiconductor lasers may also be used as laser sources. This permits heating of the desired joining area all at once.
According to an especially preferred embodiment of this method, the linear laser beam is generated by at least one semiconductor laser (diode laser). Depending on the desired line length, a diode laser or a row of several of these diode lasers arranged side by side may be used. According to another embodiment of this method, this makes it possible to adjust the line length by the distance of the laser source from the contact surface with a corresponding adjustment of the laser power for the required melting point. To do so, the laser beam is collimated in one axis (fast axis) and divergence is allowed in the other axis (slow axis). In addition, the line width of the laser beam can be kept essentially constant with a change in the distance of the laser source from the contact surface. A trapezoidal beam emitted by a diode laser having essentially a narrow elongated cross section is produced through corresponding known optical arrangements on the diode laser, so that the desired line width can be adjusted and the required energy density can be achieved by adjusting the laser power at any desired distance of the laser source from the contact surface.
An especially economical method is achieved by the fact that the relative movement between the laser beam and the workpieces is created by a mirror that can be pivoted or rotated about an axis parallel to the workpieces. The movement of this mirror can be controlled rapidly by methods known in optics, so a very short cycle time can be achieved.
In a preferred embodiment, the device according to this invention which makes it possible for this method to be carried out has a laser source, such as a diode laser, which generates a laser beam with an elongated cross section as a laser line, and according to another preferred embodiment, and it has a mirror which can be pivoted or rotated about an axis parallel to the contact surface, is arranged above the contact surface, deflects the laser beam from the laser source to the contact surface and can be operated by means of a drive; it also has a clamping device with the workpieces to be joined and the mask held in it, with the plate of the clamping device facing the laser source being made of a material that is transparent for the laser beam.